Among vehicles, there is an automobile of a hatchback type including a back door. In a vehicle body rear part of the vehicle of this type, a back door opening section having a round structure opened and closed by the back door is provided.
In the past, in the vehicle body rear part of such a vehicle, the back door opening section is formed in a closed cross-sectional structure, whereby torsional rigidity is secured (see, for example, Patent Document 1).